Method for producing synthetic quartz glass

ABSTRACT

A known method for producing synthetic quartz glass comprises the method steps: providing a liquid SiO 2  feedstock material, which contains octamethylcyclotetrasiloxane D4 as the main component, vaporizing the SiO 2  feedstock material into a feedstock material vapor, converting the feedstock material vapor into SiO 2  particles, depositing the SiO 2  particles on a deposition surface while forming a porous SiO 2  soot body, and vitrifying the SiO 2  soot body while forming the synthetic quartz glass. Starting therefrom, to produce large-volume cylindrical soot bodies with outer diameters of more than 300 mm of improved material homogeneity, it is suggested according to the invention that the liquid feedstock material contains additional components comprising hexamethylcyclotrisiloxane D3 and the linear homolog thereof with a weight fraction mD3, decamethylcyclohexasiloxane D6 and the linear homolog thereof with a weight fraction mD6, and tetradecamethylcycloheptasiloxane D7 and/or hexadecamethylcyclooctasiloxane D8 and the linear homologs thereof with a weight fraction mD7+, wherein the weight ratio mD3/mD6 is in a range between 0.05 and 90 and the weight fraction mD7+ is at least 20 wt. ppm.

FIELD OF THE INVENTION

The present invention relates to a method for producing synthetic quartzglass, the method comprising the steps of:

-   (a) providing a liquid SiO₂ feedstock material, which contains    octamethylcyclotetrasiloxane D4 as the main component,-   (b) vaporizing the SiO₂ feedstock material into a feedstock material    vapor,-   (c) converting the feedstock material vapor into SiO₂ particles,-   (d) depositing the SiO₂ particles on a deposition surface while    forming a porous SiO₂ soot body,-   (e) vitrifying the SiO₂ soot body while forming the synthetic quartz    glass.

PRIOR ART

For the production of synthetic quartz glass for commercialapplications, SiO₂ particles are produced from a silicon-containingstart substance in a CVD method (chemical gas phase deposition) byhydrolysis and/or oxidation and said particles are deposited on acarrier. A distinction can here be made between outside depositionmethods and inside deposition methods. In the outside deposition methodsSiO₂ particles are applied to the outside of a rotating carrier.Examples to be mentioned here are the so-called OVD (outside vapor phasedeposition) method, the VAD (vapor phase axial deposition) method or thePECVD (plasma enhanced chemical vapor deposition) method. The best knownexample of an inside deposition method is the MCVD (modified chemicalvapor deposition) method in which SiO₂ particles are deposited on theinner wall of an externally heated tube.

At a sufficiently high temperature in the area of the carrier surfacethe SiO₂ particles are directly vitrified (“direct vitrification”). Anexample thereof is the “boule production” described in U.S. Pat. No.5,043,002. SiO₂ particles are here deposited by means of depositionburners, which are directed from above into a rotating mold, and aredirectly vitrified so that a quartz glass body (“boule”) is verticallybuilt up from the bottom to the top in said mold.

By contrast, in the so-called “soot method” the temperature during thedeposition of the SiO₂ particles is so low that a porous soot layer isobtained that is sintered in a separate method step into transparentquartz glass. An example thereof is the OVD method known from DE 10 2007024 725 A1, in which a deposition burner is fed with combustion gases inthe form of hydrogen and oxygen and with a silicon-containing startcompound which is converted in a burner flame assigned to the depositionburner into SiO₂ particles which are deposited layer by layer withformation of a SiO₂ blank while the deposition burner is reversinglymoving along a carrier rotating about its longitudinal axis.

In a modification of the soot method which aims at higher productivity,a plurality of deposition burners are used instead of only onedeposition burner, the plural deposition burners being reversinglyreciprocated for soot deposition in a joint row of burners along therotating carrier. Each of the burner flames only sweeps over asub-length of the carrier in this process, whereby inhomogeneities mayarise in the SiO₂ soot structure at the turning points of the burnermovement.

In the final analysis, both direct vitrification and soot method yield adense, transparent synthetic quartz glass of high purity.

The formation of layer structures is inherent to said production methodson account of the layerwise deposition of SiO₂ particles. These may benoticed as so-called striae that hint at differences in the refractiveindices between neighboring layers. As a rule, a distinction can be madebetween cylindrical SiO₂ blanks with a concentric layer structure andthose with an axial layer structure. In the OVD method a layer structureis e.g. produced with a spiral layer extending substantiallyconcentrically with respect to the longitudinal axis of the blank inthat SiO₂ particles are deposited layer by layer on the cylinder outersurface of the carrier rotating about its longitudinal axis. Bycontrast, in the VAD method in which a SiO₂ solid cylinder is built upin the direction of the longitudinal axis of the cylinder on adisc-shaped rotating carrier by axial deposition, a helical layerstructure is usually obtained with axially successive layers extendingin a direction perpendicular to the longitudinal axis of the cylinder.

High demands are made on the homogeneity of the refractive indices inthe case of synthetic quartz glass used in microlithography or foroptical components in communications engineering. Therefore, for theelimination of layers in quartz glass cylinders, multi-step deformationprocesses have been suggested, for instance in DE 42 04 406 A1 and EP673 888 A1, which describes a tool-free method for homogenizing quartzglass bodies subject to striae by multidimensional compression andelongation of a softened quartz glass mass. These methods are efficient,but time-consuming and cost-intensive.

In the past silicon tetrachloride (SiCl₄) turned out to be useful as thesilicon-containing feedstock material. SiCl₄ and otherchlorine-containing substances already exhibit high vapor pressures atmoderate temperatures below 100° C., so that possible impurities remainin the liquid phase and the manufacture of high-purity soot bodies isfacilitated. On the other hand, hydrochloric acid which causes highcosts in terms of flue gas scrubbing and disposal is formed during thereaction of SiCl₄ and other chlorine-containing feedstock materials.

Therefore, a multitude of chlorine-free feedstock materials have beentested for quartz glass production. Monosilanes, alkoxysilanes andsiloxanes should be mentioned as examples. A particularly interestinggroup of chlorine-free feedstock materials is formed by thepolyalkylsiloxanes (also shortly “siloxanes”), which are e.g. known fromEP 463 045 A1. The substance group of siloxanes can be subdivided intoopen-chain and closed-chain polyalkylsiloxanes. The polyalkylsiloxaneshave the general sum formula Si_(p)O_(p)(R)_(2P), where P is an integer≧2. The residue “R” is an alkyl group, in the simplest case a methylgroup.

Polyalkylsiloxanes are distinguished by a particularly high content ofsilicon per weight fraction, which contributes to the economy of theiruse in the manufacture of synthetic quartz glass. At the momentoctamethylcyclotetrasiloxane (OMCTS) is preferably used because of itslarge-scale availability in a high purity. This substance is alsodesignated as “D4” according to a notation introduced by GeneralElectric Inc., where “D” represents the group [(CH₃)₂Si]-0-.

However, on account of the relatively high boiling temperature and thechemical similarity with other polyalkylcyclosiloxanes, such ashexamethylcyclotrisiloxane (D3), decamethylcyclopentasiloxane (D5),dodecamethylcyclohexasiloxane (D6) and tetradecamethylcycloheptasiloxane(D7), the purification of D4 requires a time-consuming and expensivedistillation procedure.

U.S. Pat. No. 5,879,649 A is concerned with such a purification ofpolyalkylsiloxanes as feedstock material for quartz glass production.The publication suggests a two-step distillation process using a carbonfilter and a molecular sieve to limit the amount of impurities withboiling temperatures above 250° C. to an amount of less than 14 wt. ppm,preferably to less than 2 wt. ppm. Typically, these high-boilingimpurities are components having a mean molecular weight of more than500 g/mole. It is reported that agglomerates of such high-boilingimpurities lead to clogging in the gas supply system and effect “gelformation”, thereby producing defects in the quartz glass. The defectmechanism seems here to be the deposition of undecomposed orincompletely decomposed high-boiling agglomerates that will subsequentlydecompose with release of gases and may lead to bubbles in the quartzglass.

The suggested two-step purifying method is complicated and expensive,and it has been found that, even in the case of an optimized processcontrol, material inhomogeneities arise particularly in the form of thinlayers of increased density in the quartz glass.

As an alternative thereto, DE 103 02 914 A1 suggests for the productionof synthetic quartz glass with a favorable damaging behavior towardsshort-wave UV radiation that a mixture of SiCl₄ and of an oligomericsilicon compound containing plural Si atoms, such as e.g. a siloxane,should be used as the feedstock material.

The silicon-containing feedstock material can be supplied in liquid formto the consumer, such as e.g. a deposition burner. As a rule, however,the liquid feedstock material is converted by means of a vaporizer intoa gaseous or vaporous phase and supplied to the consumer as a continuousgas stream.

A large number of vaporizers are known. A known vaporizer includes acontainer (a so-called “bubbler”) in which the liquid feedstock materialis kept and heated by means of a heater to a temperature around thevaporization temperature. A carrier gas is passed through the heatedliquid and loaded in this process with the vaporizing feedstock materialand supplied under pressure via a pipe system to the reaction zone. Thecarrier gas is e.g. oxygen. Such a supply of media to a depositionburner for producing synthetic quartz glass by direct vitrification ise.g. described in EP 908 418 A1.

The vaporization rate of the feedstock material depends substantially onthe temperature and the residence period of the carrier gas in theliquid phase. Both parameters are influenced by the height of the liquidcolumn and by the supply rate and the flow velocity of the carrier gas.For instance, the size of the carrier gas bubbles in the liquid columnhas impacts on their rate of ascent in the liquid and thus on theloading with the feedstock material and on the vaporization rate.Changes in the liquid amount have also an impact on the heat transfer.These complex interactions can be handled in the easiest way in thatliquid feedstock material is constantly delivered so that the liquidlevel in the bubbler will not decline.

However, even at a constant liquid level, impurities of a relativelyhigher boiling point may gradually enrich in the liquid phase withformation of a “sump”, so that the composition of the feedstock materialarriving at the deposition burner will change over time.

An additional problem will arise in soot deposition methods in whichseveral consumers have to be simultaneously fed with the feedstockmaterial, e.g. in the case of a soot deposition with a multiple burnerassembly. To avoid an irregular soot deposition and layer formation, itis here particularly important that each deposition burner has the samesoot build-up characteristics in terms of quantity and quality.Variations in the gas supply of the individual deposition burners can beminimized in that these are fed from a common feed tank via a “flowdistributor”, as is e.g. described in DE 195 01 733 A1. This, however,requires a complicated infrastructure of the media supply.

Another type of vaporizer, as is e.g. described in U.S. Pat. No.5,356,451 A1, avoids the above-mentioned “sump formation”. In thisvaporizer a liquid reservoir for the feedstock material is providedinside a vaporization chamber, said reservoir extending along alongitudinal side of the chamber and being filled continuously. If theliquid level exceeds a predetermined overflow height, the liquid willflow off via U-shaped channels out of the storage chamber on the longside towards an inclined plane, thereby forming a thin film thereon. Thechamber is heated such that the liquid film has completely vaporized onthe inclined plane before the lower end of the inclined plane isreached. It is thereby ensured that the whole liquid, including allimpurities, vaporizes at a higher boiling point.

An embodiment of a so-called vertical vaporizer is known from DE 24 35704 A1. It is suggested therein that the liquid to be vaporized shouldbe heated, supplied to a vertically standing rotation-symmetricalcontainer and sprayed radially against the inner wall of the containerwith formation of droplets having a diameter of less than 6 mm and thatthese droplets should be deposited at said place. The vaporous productsare removed upwards while the non-vaporized liquid collects in the lowerpart of the container where it can be removed continuously or from timeto time.

The known method works like an additional distillation step duringvaporization, wherein constituents of low volatility can be removed viathe sump. This results in a higher purity of the vaporized portion ofthe feedstock material. Yield and throughput are however comparativelylow.

Technical Objective

In general, the structure of a SiO₂ soot body is sufficientlygas-permeable, which facilitates a uniform gas-phase treatment orsintering. In the area of layers of increased density, this is onlypossible to a limited degree. The reason is that the layers representdiffusion barriers which may produce a non-uniform treatment result indrying or sintering processes. These problems arise because of longdiffusion paths, especially with large-volume SiO₂ soot bodies. Layerareas can particularly be accompanied by a locally increased content ofhydroxyl groups and possibly of chlorine.

These inhomogeneities in the material of the soot body manifestthemselves in the quartz glass body made therefrom, e.g. in the form ofaxial, radial or azimuthal variations of the hydroxyl-group or chlorineconcentration or the viscosity values, and lead to unfavorableproperties in the end product, such as e.g. an unfavorable damagingbehavior with respect to radiation with high-energy UV light.

It is the object of the present invention to provide a method forproducing SiO₂ soot bodies of high material homogeneity, particularlylarge-volume cylindrical soot bodies with outer diameters of more than300 mm.

General Description of the Invention

This object, starting from the aforementioned method, is achievedaccording to the invention in that the liquid feedstock materialcontains additional components comprising hexamethylcyclotrisiloxane D3and the linear homolog thereof with a weight fraction mD3,decamethylcyclohexasiloxane D6 and the linear homolog thereof with aweight fraction mD6, and tetradecamethylcycloheptasiloxane D7 and/orhexadecamethylcyclooctasiloxane D8 and the linear homologs thereof witha weight fraction mD7+, wherein the weight ratio mD3/mD6 is in a rangebetween 0.05 and 90 and the weight fraction mD7+ is at least 20 wt. ppm.

In contrast to the known methods in which a feedstock material is usedthat consists of a single defined silicon compound of the highest puritypossible, the present invention suggests a SiO₂ feedstock material thatis present as a mixture of different polyalkylsiloxanes.Octamethylcyclotetrasiloxane will here and in the following be called D4in short, and other polyalkylcyclosiloxanes are consequently called D3,D5, D6, D7 and D8. D4 forms the main component of the mixture. Inaddition to D4, the mixture contains chemically similarpolyalkylsiloxanes, namely those with a smaller molecular mass than D4(including D3) and also those with a greater molecular mass than D4(including D6 and D7). Thus the additional constituents of the feedstockmaterial which are summarized under the term “additional components”have molecular masses and boiling temperatures that differ both upwardsand downwards from the relative molecular mass of D4 (about 297 g/mol)and in the boiling temperature of D4 (about 175° C.).

The liquid SiO₂ feedstock material which is enriched with additionalcomponents is supplied to a reaction zone in gaseous form and isdecomposed in this process by oxidation and/or hydrolysis and/orpyrolysis into SiO₂. The reaction zone is e.g. a burner flame or plasma.In the reaction zone the polyalkylsiloxane molecule is oxidativelydecomposed step by step into SiO₂, resulting in a SiO₂ primary particleto which other SiO— or SiO₂ molecules from the gas phase are attached.The attachment process will end on the way through the reaction zonetowards a deposition surface as soon as the agglomerated or aggregatedSiO₂ particles enter into a zone in which there are no longer any otherundecomposed molecules available.

These separation, oxidation and attachment processes (hereinafter alsosummarized under the term “particle formation process”) take place withdifferent kinetics and at different temperatures, depending on themolecular mass and size of the polyalkylsiloxane molecule.

In the method of the invention the SiO₂ feedstock material contains atleast one additional component with a smaller relative molecular massthan D4 and also at least one additional component with a greaterrelative molecular mass than D4. It is assumed that during thehydrolytic or pyrolytic decomposition of the polyalkylsiloxanes theactivation energy of the oxidative attack or the thermal energy neededfor separation is increasing with an increasing molecular mass. As aconsequence, agglomerates and aggregates of different sizes with abroader particle size distribution than in monodisperse SO₂ feedstockmaterial are formed in the reaction zone.

A possible explanation for this broadening of the particle sizedistribution is that due to the different gas phase kinetics one alsoobtains different kinetics with respect to the formation of SiO₂ primaryparticles on which further growth can take place due to the attachmentof further SiO₂ molecules from the gas phase. Another possibleexplanation is that already the number and configuration of the siliconatoms of the respective polyalkylsiloxane molecule predetermine the sizeof the SiO₂ primary particles and thus with this also the size of theSiO₂ particles evolving therefrom and their concentration in thereaction zone.

At any rate, in comparison with the use of pure D4, the additionalcomponents in the method according to the invention change the particleformation process towards a broader spectrum of the sizes of theevolving SiO₂ particles, accompanied by a change in the soot bodystructure.

Due to the manufacturing process soot bodies show a certain layerstructure, the layers representing regions of local changes in thedensity or the chemical composition. Normally, soot bodies have adensity of 25-32% based on the density of quartz glass. These sootbodies show a relative variation in the density of 3% to 4%. Thesedensity variations manifest themselves during vitrification of the sootbody in the quartz glass body and lead there to radial, azimuthal andaxial variations of the hydroxyl groups or chlorine concentrations whichmay lead to unfavorable properties of the quartz glass cylinder and thequartz glass fibers produced therefrom.

It has been found that the use of a SiO₂ feedstock material according tothe invention yields a SiO₂ soot body with a surprisingly highhomogeneity, particularly with a uniform and weak characteristic of thelayer structure.

This effect can be ascribed to the fact that the broadening of the sizedistribution of the formed and depositing SiO₂ particles either leads toa more homogeneous deposition of the SiO₂ soot than in monodisperse SiO₂particles or that it facilitates a more uniform densification of theSiO₂ soot during soot body production or during vitrification.

To achieve this effect, the following boundary conditions have to bemet:

-   (a) The feedstock material arriving in the reaction zone must    contain additional components with molecular masses at both sides of    the reference molecular mass of D4, ideally D3 and D6 and D7 or the    corresponding open-chain homologs.-    The additional components D3 and D6 are molecules with a similar    molecular mass and with a chemical similarity with respect to D4,    resulting in a moderate change in the reaction zone and the particle    formation process, accompanied by a minor flattening of the particle    size distribution.-   (b) A distribution of the additional components at both sides of D4    that is as uniform as possible. This condition will be satisfied to    a sufficient degree if the ratio mD3/mD6 of the weight fractions of    D3 and D6 is between 0.05 and 90.-    A haphazardly mixed polyalkylsiloxane mixture with weight fractions    of D3 and D6 outside said range yields poorer results with respect    to the homogeneity of the soot body.-   (c) A minimum amount of a polyalkylsiloxane boiling at an even    higher temperature than D6, ideally D7 or the even longer-chained    polyalkylsiloxane D8, with a weight fraction mD7+ of at least 20 wt.    ppm in total (the term mD7 is hereinafter used for designating the    weight fraction of D7, and the term mD7+ designates the sum of the    weight fractions of D7 and D8 in the following).-    The boiling point of D7 at atmospheric pressure is about 276° C.    and thus clearly above the boiling point of D4 (about 175° C.). Due    to the great difference in the respective boiling temperatures    already small amounts of the additional component D7 have a    noticeable impact on the particle formation process. At weight    fractions mD7+ of less than 20 ppm, however, no significant effect    can be detected. With the weight fraction mD7+ an amount of D7 may    be replaced by a corresponding amount of D8. Due to their large    molecular mass in comparison with D4, D8 and its linear homolog,    however, bring about a noticeable change in the reaction zone and    the particle formation process already at a low concentration, so    that the weight fraction of D8 is preferably not more than 20 wt.    ppm of the feedstock material.

The method according to the invention is distinguished in that the knowndensity variations in the soot body are reduced. Since this reduction ofthe density variations has a direct effect on the quality of the quartzglass, more homogeneous quartz glass bodies of higher quality are thusobtained in a reproducible manner, which reduces material rejects.

With the help of the method according to the invention it is possible toobtain SiO₂ soot bodies and quartz glass bodies that exhibit aparticularly high material homogeneity which exceeds the presentlycommon standard. The advantages of the method can be explained in that astronger variation, which takes place on a microscopic scale, in thesizes of the produced SiO₂ particles, viz. the broadening of theparticle size distribution, results in a reduction of the variationwidth of the macroscopically measured density within the SiO₂ soot body.

Within the scope of the invention the term “weight fraction” describes arelative value that ensues from the mass of the respective additionalcomponent in relation to the total mass of the liquid SiO₂ feedstockmaterial. The weight ratio mD3/mD6 of the two weight fractions of theadditional components D3 and D6 is thus a dimensionless value that canbe determined by division of the two relative weight fractions.

Within the scope of the invention the term polyalkylsiloxane comprisesboth linear (including also branched structures) and cyclic molecularstructures. It is however preferred when the polyalkylsiloxanes of theSiO₂ feedstock material comprise at least threepolymethylcyclosiloxanes, selected from the group consisting ofhexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4),dodecamethylcyclohexasiloxane (D6). The notation D3, D4, D6 derives froma notation introduced by General Electric Inc., wherein “D” representsthe group [(CH₃)₂Si]-0-. The main component is D4 with a portion of atleast 70% by wt., particularly at least 80% by wt., preferably at least90% by wt., particularly preferably at least 94% by wt., each time basedon the total weight of the feedstock material.

The liquid feedstock material can be produced by mixing the individualcomponents or by blending component mixtures, or by premature removalfrom a distillation column or premature termination of a distillation,as is otherwise standard for the purification ofoctamethylcyclotetrasiloxane. Hence, in the method according to theinvention the purification of octamethylcyclotetrasiloxane can beomitted and, instead of this, a less pure and thus also cheaper D4quality can be used. The similar densities of the polyalkylsiloxanes ofthe feedstock material counteract a separation. In a preferredconfiguration of the method, the ratio mD3/mD6 is in the range between0.1 and 40.

It has been found that the addition of D3 and D6 to the SiO₂ feedstockmaterial, which consists mainly of D4, in this comparatively small rangeof the mass ratio achieves a particularly significant reduction of theinhomogeneities in the density of the quartz glass mass.

In this context it has also turned out to be advantageous when theweight fraction mD7+ is in the range between 30 wt. ppm and 100 wt. ppm.

Apart from D6, D7 and D8, decamethylcyclopentasiloxane D5 and its linearhomolog represent polyalkylsiloxanes of a higher molecular weight thanD4. The effect of D5 with respect to the particle size broadening in theparticle formation process and the accompanying equalization in the SiO₂soot structure is however smaller than that of D6 or D7, but it makes asupplementary contribution.

To enable the additional components to produce an optimal effect on theparticle size broadening in the particle formation process and theequalization in the SiO₂ soot structure, a minimum amount of saidcomponents is advantageous. Therefore, it has turned out to beadvantageous when the sum of the weight fractions mD3+mD6 is in therange between 200 wt. ppm and 20,000 wt.-ppm, preferably between 500 wt.ppm and 15,000 wt. ppm.

Surprisingly, it has been found that the additional components D3 and D6need not dominate the SiO₂ feedstock material, but admixtures are enoughfor providing a SiO₂ feedstock material that during deposition in theSiO₂ soot method leads to a broadening of the SiO₂ particle size andthus to an enhanced homogeneity of the soot body. Particularly goodresults are achieved up to an upper limit of not more than 10% of thetotal weight. On the other hand, already small amounts of additionalcomponents D3 and D6 have the effect that density changes in the sootbody are covered by different particle sizes of the SiO₂ feedstockvapor, so that a more homogeneous soot body or quartz glass body isobtained on the whole. At weight fractions of less than 0.5% by wt. ofthe additional components D3 and D6, the effect is however no longersignificantly noticeable.

The activation energy needed for the decomposition is lower in thepolyalkylsiloxane D3 than in D4, which might be due to the fact that theD3 ring can be opened more rapidly and more easily due to the higherring tension than the more stable D4 ring. It also emerges that thedecomposition of the polycyclosiloxanes D6 and D5 also requires a higheractivation energy than D4 to trigger a thermal separation of thecorresponding ring molecules. On the whole, however, it has been foundthat the energy difference between D4 and D3 is greater than between D4and D6.

For this reason and since D3 shows a greater tendency towardspolymerization reactions, it has turned out to be advantageous when theamount of the additional component D3 is smaller than that of theadditional component D6. Thus an advantageous configuration variant ofthe method according to the invention is distinguished in that mD3 is inthe range between 200 wt. ppm and 15,000 wt. ppm and that mD6 is in therange between 50 wt. ppm and 2,000 wt. ppm.

In the above-mentioned quantitative proportions of the additionalcomponents, density variations within the soot body of less than 0.4%can be achieved. During the transition of the liquid feedstock materialinto the gas phase the composition of the liquid may change due todecomposition, polymerization or distillation. This happens especiallywhen, like in the known vaporization systems, the liquid to be vaporizedis brought into contact with a hot surface. Hot surfaces may lead inorganic feedstock material to unforeseeable changes, such asdecompositions or polymerizations. This results in a certain variabilityand non-reproducibility in the process control, which may lead todefects in the particle formation process and to inhomogeneities in thesoot structure. This will be particularly noticed when, like in thepresent case, exact compositions of components, which are very similarto one another in terms of chemistry, are important in the particleformation process. Moreover, the known vaporizer systems pose the riskthat ultrafine liquid droplets are entrained together with thedischarged vapor stream, which may also lead to material inhomogeneitiesin the soot body.

To avoid such effects or to keep them as small as possible, avaporization method is preferably used in which the vaporizing processcomprises the following steps:

-   -   heating the SiO₂ feedstock material,    -   introducing the heated SiO₂ feedstock material into an expansion        chamber, so that at least a first part of the SiO₂ feedstock        material vaporizes due to a pressure drop,    -   mixing the SiO₂ feedstock material with a heated diluent, so        that at least a second part of the SiO₂ feedstock material        vaporizes due to a decrease in the dew point.

In this two-stage vaporizing process the liquid SiO₂ feedstock materialis atomized in the expansion chamber into fine droplets which arecompletely vaporized without any contact with solid, particularlymetallic, surfaces. Thus it is possible to convert the compositionprovided in the liquid SiO₂ feedstock material, including weight ratiomD3/mD6 and mD7+, into the gaseous SiO₂ feedstock vapor. Therefore, oneobtains congruent proportions of the additional components in the liquidSiO₂ feedstock material and in the SiO₂ feedstock vapor, hereinafteralso called “feedstock vapor” in short.

Hence, this procedure satisfies two principal requirements. On the onehand, the provided liquid SiO₂ feedstock material is converted ascompletely as possible (i.e. at least 99% by wt.) into the gas phase. Onthe other hand, this vaporization process is configured such that theSiO₂ feedstock material vaporizes while maintaining the proportionalcomposition of the components. In particular, the mass ratio of theadditional components is to be substantially congruent in the liquidphase and also in the gas phase.

Within the scope of the invention the statement that vaporization takesplace congruently while maintaining the weight ratios refers to theratio mD3/mD6 in the liquid and in the solid phase. The ratio

τ=(G_liquid−G_vapor)/G_liquid

should thus be not more than ±500 ppm, preferably not more than ±100ppm, wherein G_liquid is the weight ratio mD3/mD6 in the liquid SiO₂feedstock material, and G_vapor is the weight ratio mD3/mD6 in thegaseous SiO₂ feedstock material.

A diluent is used for vaporizing the feedstock material. Advantageously,the diluent is a carrier gas which flows through the expansion chamber.For this reason the term diluent gas and the term carrier gas will beused as synonyms hereinafter. The partial pressure of the liquid SiO₂feedstock material in the expansion chamber is reduced by supplying thediluent, whereby the dew point thereof is lowered, so that a possiblepreliminary heating of the SiO₂ soot body need not be strong to ensure acomplete conversion of the SiO₂ feedstock material into the SiO₂feedstock vapor.

Ideally, the liquid feedstock material is completely converted into thegas phase. A vaporization degree of at least 99% by wt., preferably atleast 99.9995% by wt., of the liquid SiO₂ feedstock material enteringthe vaporization process is however acceptable. This can be achieved bymeans of the two-stage vaporization process, i.e. the combination of thevaporization due to a pressure drop and a decrease in the dew point. Ofthe SiO₂ feedstock material introduced as a liquid into the vaporizer,only very small fractions, preferably less than 20 ppm, preferably lessthan 10 ppm, particularly preferably less than 5 ppm, do not vaporize.In individual tests the fraction of non-vaporized SiO₂ feedstockmaterial could even be reduced to less than 2.5 ppm.

The “dew point” is here the temperature at which an equilibrium statebetween condensing and evaporating liquid is obtained.

In an advantageous configuration of this procedure it is intended thatintroducing the heated SiO₂ feedstock material into the expansionchamber comprises an injection phase in which the SiO₂ feedstockmaterial is atomized in liquid form into fine droplets, the dropletshaving a mean diameter of less than 5 μm, preferably of less than 2 μm.

The liquid feedstock material is transformed into a cloud of smalldroplets which can evenly vaporize due to expansion as a result of thepressure drop. The fine droplets provide a large surface that allows arapid and efficient and above all complete vaporization of the liquidfeedstock material before it gets into contact with hot surfaces of thevaporizer. Changes in the composition due to decomposition,polymerization or distillation are thereby avoided to a substantialdegree, so that a defined composition of the feedstock material suppliedto the consumer as well as a reproducible particle formation process areensured.

The combination according to the invention of providing a composition ofthe feedstock material optimized for homogeneous soot formation, on theone hand, and the maintenance thereof during vaporization and the supplyto the reaction zone, on the other hand, results in a soot body fromwhich a quartz glass that is distinguished by high material homogeneityand radiation resistance is obtained by sintering or vitrification.

To support the division of the liquid into small droplets, ultrasonicatomizers are in principle suited, in the case of which due to theaction of ultrasound a uniform and fine atomization of the SiO₂feedstock material is induced. Within the scope of the inventionultrasound designates a sound with frequencies between 16 kHz and 1.6GHz. In an ultrasonic atomizer the liquid is atomized without pressureapplication and without heating. For instance, a piezoceramic wettedwith the liquid can be induced to vibrate by means of high-frequencyalternating voltage. As a consequence, ultrasonic waves form in theliquid, the maximum strength of said waves being reached at a specificliquid level and said waves effecting the formation of a so-calledultrasonic trunk. Small liquid droplets or aerosols detach from thisultrasonic trunk and can be used for the desired application. Theadvantage of ultrasonic atomizers lies in the uniform atomization ofvariable volume flows, the almost constant drop spectrum over the wholevolume flow range and the drops' own small velocity, resulting in a goodcontrollability of the jet. Thus, a narrow droplet size distribution canbe accomplished in a reproducible manner through ultrasonic atomization,which has a positive effect on the uniformity of the vaporizationresult.

As an alternative, the conversion of the liquid feedstock material intothe vapor phase takes place exclusively by use of the carrier gas, withthe consequence of the reduction of the partial pressure.Correspondingly sufficient amounts of the diluent/carrier gas must hereflow through the expansion chamber and thereby ensure vaporization ofthe liquid SiO₂ feedstock material.

As a third and particularly preferred alternative, the liquid feedstockmaterial is vaporized both by using the pressure drop and by loweringthe partial pressure. This variant has turned out to be particularlyadvantageous when large amounts of liquid feedstock material have to bevaporized for obtaining large-volume quartz glass cylinders (e.g. withdiameters of more than 250 mm). To convert the needed large feedstockmaterial amounts in a gentle and uniform manner from the liquid phaseinto the gas phase, it has turned out to be advantageous when a SiO₂feedstock material which is overheated at least for partial componentsis introduced into the expansion chamber and is there transferred bypressure drop and by use of a diluent into the gas phase. Thiscombination of the vaporization process due to a pressure drop and areduction of the dew point makes it possible that of the SiO₂ feedstockmaterial introduced into the vaporizer in a liquid form, only very smallamounts do not vaporize.

The liquid SiO₂ feedstock material can be transferred more easily anduniformly into the gas phase if the individual amounts to be vaporizedare each small and have a large surface. This can optimally be achievedin that the liquid of the SiO₂ feedstock material is atomized into finedroplets. The finer the droplets, the more efficient, complete and rapidis the vaporization process. The atomized droplets can then betransferred via pressure drop and/or blending with the heateddiluent/carrier gas into the gas phase.

It has turned out to be advantageous when the contact of the finedroplets with the hot carrier gas takes place in a chamber that is keptat a temperature ranging from 150° C. to 230° C. At a temperature ofless than 150° C. there is a certain risk that droplets do not vaporizecompletely, so that liquid is entrained into the reaction zone, whichleads to inhomogeneities in the particle formation process and todefects in the soot body structure, such as bubbles. At a temperatureabove 230° C., otherwise energetically inhibited reactions withnon-reproducible and undesired reaction products, particularlydecompositions and polymerization reactions, are more likely.

The size range of the droplets depends on many parameters. Apart fromthe rheological properties of the liquid and the geometry of theatomizing nozzle, this is particularly the exit velocity of the liquidout of the atomizing nozzle, which is substantially defined by thepressure difference. Within a pressure difference range of 1.2 to 1.8bar the exiting liquid jet disintegrates into fine drops with aparticularly narrow drop size distribution on account of a turbulentflow.

Preferably, a carrier gas is used that contains nitrogen, argon orhelium.

These are gases that show an inert behavior with respect topolyalkylsiloxanes, so that oxidation, polymerization or decompositionreactions between the liquid and the carrier gas, particularly underpressure and at an elevated temperature, and thus a non-reproduciblechange in the composition of the feedstock material are avoided.

A further configuration variant of the method according to the inventionis distinguished in that upon introduction of the SiO₂ feedstockmaterial into the expansion chamber a composition of the SiO₂ feedstockmaterial is measured by means of a concentration detector.

Within the scope of this configuration the composition, i.e. e.g. theratio D3, D6 and D7, is directly monitored at the expansion chamber. Thesupplied SiO₂ feedstock material is here analyzed by means of theconcentration detector, e.g. a gas chromatographer. Such an analyzingdevice with a concentration detector can also be arranged at the outletof the expansion chamber and can determine the composition of the SiO₂feedstock vapor. One or both detectors may be part of a qualitymanagement system and send the composition measurement to a computingsystem where the quality of the added materials and vapors is monitored.Such a permanent monitoring of the composition and the weight fractionsof the SiO₂ feedstock material ensures that the method according to theinvention can be employed at any time and soot bodies can thereby bebuilt up having a variance of the density variations of onlyδ_(Inv)=0.023%. Extremely homogeneous synthetic quartz glasses can thenbe produced by vitrification from soot bodies of such a structure.

The liquid feedstock material can then be produced by mixing theindividual components D3, D4, D5, D6 and D7 or by blending componentmixtures. Preferably, providing a liquid SiO₂ feedstock materialaccording to method step (a) comprises:

-   -   providing a first mixture of polyalkylsiloxanes in which D3, D4,        D5, D6 and the linear homologs thereof account for at least 99        wt. %,    -   providing a second mixture of polyalkylsiloxanes in which D7        accounts for an amount of at least 5,000 wt. ppm, preferably at        least 10,000 wt. ppm, particularly preferably at least 20,000        wt. ppm,    -   producing a blend from first and second mixture of        polyalkylsiloxanes prior to or during vaporization according to        method step (b) in a mixing ratio leading to a weight fraction        for mD7+ of at least 20 wt. ppm in the total mixture.

A polyalkylsiloxane mixture in the sense of the first mixture, based onD4 and with D4 and D6 in the ratio and in the amount as desired, cane.g. be obtained in that a standard distillation process is used for thepurification of D4. This distillation process can then be run untilpurification of D4, which however is not necessary—also for reasons ofcosts—because additional components are removed from the mixture, ifnecessary, and these must again be added by means of the second mixture.In the total mixture the first mixture accounts for the greatestfraction, e.g. more than 95% by wt. As a rule, the quantitative amountmD7+ is however too small in such mixtures.

The second mixture primarily serves to set the predetermined amount ofD7 (and/or D8) and possibly to correct the ratio mD3/mD6.Polyalkylsiloxane mixtures with a high amount of D7 and longer-chainedmolecules are e.g. present in an intermediate fraction which in amulti-stage conventional fractionizing distillation apparatus can beremoved before the last distillation step. The weight fraction mD7+ insuch intermediate fractions is preferably at least 5000 wt. ppm. Thenecessary blending amount of the second mixture depends essentially onthe mD7+ weight fraction and is small in comparison with the blendingamount of the first mixture. An expensive purification of D4 or the useof high-purity D4 can be omitted at any rate.

The blending of first and second mixture takes place in the liquid phaseof the feedstock material. As an alternative, the first and the secondmixture are first vaporized and the mixing process is carried out bycombining the vapor flows before or during method step (c). Due to theuse of separate vaporizers for different components of the feedstockmaterial the vaporization parameters, such as vaporization temperatureand vaporization rate, can be individually adapted to the components tobe vaporized and they can be optimized.

In a preferred method variant, it is intended that the SiO₂ particlesare deposited on the deposition surface according to method step (b) inthat the feedstock material vapor is supplied to a plurality ofdeposition burners arranged in a burner row, which are moved jointlyreversingly along the deposition surface.

In this known method of producing SiO₂ soot bodies the depositionburners are reciprocated along a sub-section of the soot body surface.Due to the differences in the geometry of the individual depositionburners or in the media supply to the burners, this procedure isparticularly prone to density variations in the soot body, particularlyalso in the overlap region of neighboring deposition burners. It hasbeen found that the soot density homogeneity can be improved,particularly in the area of the turning points of the burner movement,which can be ascribed to the characteristic of the method according tothe invention to rather blur than allow soot density differences onaccount of the broadening of the particle size distribution in theparticle formation process.

The method according to the invention is therefore particularly wellsuited for producing synthetic quartz glass that is used for theproduction of optical fibers. Small density variations of the soot bodyfacilitate the production of high-quality quartz glass that isparticularly suited for optical fibers for telecommunications.

PREFERRED EMBODIMENT

The invention will now be explained in more detail with reference toembodiments and a drawing, in which:

FIG. 1 shows a device for performing the method according to theinvention for producing a SiO₂ soot body, in a schematic illustration;

FIG. 2 shows a diagram regarding the layer density in soot bodies independence upon the composition of the feedstock material;

FIG. 3 is a CT image of a soot tube produced according to the inventionby using a feedstock material from different polyalkylsiloxanes, in aview taken in the direction of the longitudinal axis of the soot tube;

FIG. 4 shows, by way of comparison, a CT image of a SiO₂ soot tubeproduced on the basis of the prior art by using a pureoctamethylcyclotetrasiloxane as feedstock material;

FIG. 5 is a schematic diagram showing the various elements of thequartz-glass producing system according to the invention;

FIG. 6 is a schematic illustration of a vaporization chamber;

FIG. 7 shows a constitutional formula of the molecule D3; and

FIG. 8 shows a constitutional formula of the molecule D7.

The device shown in FIG. 1 serves to produce a SiO₂ soot body 200. Aplurality of flame hydrolysis burners 140 which are arranged in a roware disposed along a carrier tube 160 of aluminum oxide and arereversingly reciprocated for soot deposition in a joint burner row alongthe rotating carrier tube 160, each of the burner flames 143 onlysweeping over a sub-length of the carrier tube 160. The flame hydrolysisburners 140 are mounted on a joint burner block 141 which isreciprocated in parallel with the longitudinal axis 161 of the carriertube 160 between two turning points that are stationary with respect tothe longitudinal axis 161, and which is displaceable in a directionperpendicular thereto, as outlined by the directional arrows 142. Theburners 140 consist of quartz glass; their center-to-center distance is15 cm.

The burner flame 143 assigned to each of the flame hydrolysis burners140 constitutes a reaction zone within the meaning of the invention. Theflame hydrolysis burners 140 are each fed with oxygen and hydrogen asburner gases and with a SiO₂ feedstock vapor as feedstock material forthe formation of the SiO₂ particles, the SiO₂ feedstock vapor 107containing a mixture of vaporized polyalkylsiloxanes with thecomposition as can be seen in Table 1. The liquid SiO₂ feedstockmaterial 105 which is enriched with additional components is vaporizedand supplied to the reaction zone in a gaseous form and is decomposedtherein by oxidation and/or hydrolysis and/or pyrolysis into SiO₂particles.

The SiO₂ particles themselves are present in the form of agglomerates oraggregates of SiO₂ primary particles with particle sizes in thenanometer range. The SiO₂ particles are deposited on the cylinder outersurface of the carrier tube 160 rotating about its longitudinal axis161, so that the soot body 200 is built up layer by layer with an outerdiameter of 350 mm. During the deposition process, a temperature ofabout 1200° C. is established on the soot body surface.

The SiO₂ feedstock vapor 107 with the polyalkylsiloxane mixture isproduced by means of a vaporizer system 120 that comprises a storagecontainer 110 for the liquid mixture, a liquid pump 122, a flow meter123 for liquid, an MFC (mass flow controller) 124 for the controlledsupply of a carrier gas stream 152 of nitrogen, and a double-walledheatable vaporization chamber 125 with an atomizer 128. The storagecontainer 110, a pump 122 and an atomizer 128 are interconnected bymeans of flexible metallic lines. The storage container 110 is heated toa temperature of 170° C., and the heated liquid is supplied by means ofthe pump 122 via the flow meter 123 in an exact dosage to the atomizer128. A concentration detector may here be provided in the connectionline between the flow meter 123 and the atomizer 128 for monitoring thecomposition of the SiO₂ feedstock material 105 and/or the SiO₂ feedstockvapor 107.

The atomizer 128—also called atomizing nozzle—is an ultrasonic atomizer.Said atomizer is simultaneously fed with a nitrogen carrier gas streamvia the MFC 123 at a pressure of 1.5 bar to 5 bar and with the liquid tobe vaporized. Said liquid is atomized into fine droplets with a maximumdiameter of 1 μm and a small droplet size distribution with a meandiameter (d₅₀ value) of 0.7 μm and directly sprayed into thevaporization chamber 125 in this process.

The vaporization chamber 125 has an internal temperature of 195° C., sothat the fine liquid droplets vaporize immediately and the vapor streamis supplied to a stationary flow distributor and distributed by saiddistributor via heat-insulated flexible media supply lines over theindividual deposition burners 140. A feed line for the combustion gasesoxygen and hydrogen and one for an auxiliary gas (oxygen), which is usedin the burner flame 143 between the stream of the feedstock material andthe stream of the combustion gas and which counteracts premature mixing,also terminate in the flow distributor. Thus combustion gases and SiO₂feedstock vapor 107 are only mixed in the hot zone of the burner flame143.

After completion of the deposition process a tube of porous SiO₂ soot(soot tube) is obtained which is subjected to a computed tomography (CT)analysis. The soot tube 200 is irradiated over its length with X-rays.The images obtained thereby permit quantitative and qualitativestatements on the intensity and homogeneity of the axial and radiallayer structure of the soot tube 200.

TABLE 1 mD3 + mD3/ No. mD3 mD6 mD7+ mD6 mD3/mD6 mD7+ “S” 1 4000 100 254,100 40 160 ++ 2 500 4,500 20 5,000 0.1 25 0 3 13,000 3,000 50 16,0004.3 260 ++ 4 200 300 80 500 0.7 3 + 5 43,000 500 110 43,500 86.0 391 0 632,000 250 120 32,250 128 267 − 7 37,000 70 180 37,070 529 206 − 856,000 130 120 56,130 430 467 − 9 41,000 300 430 41,300 136 95 −− 1090,000 100 240 90,100 900 375 −− 11 43,000 500 15 43,500 86.0 2866 −− 1213,000 3,000 15 16,000 4.3 867 − 13 200 4500 25 4700 0.04 8 −

All concentrations of the polyalkylsiloxanes in Table 1 are indicated inwt. ppm, with the remaining fraction consisting each time of D4 and thechemically similar D5 and of unavoidable impurities.

The parameter “S” is a qualitative measure of the homogeneity and theintensity of the layer structure detected by way of the CT measurement.The symbols of the qualitative assessment are as follows:

-   -   ,,++” very good,    -   ,,+” good,    -   ,,0” acceptable,    -   ,,-” poor and    -   ,,--” very poor.

Table 1 lists different compositions of the liquid SiO₂ feedstockmaterial that were examined in experiments, as well as the qualityresults that were determined on the respective soot tube.

The liquid SiO₂ feedstock material was produced from polyalkylsiloxanemixtures. The first start mixture is commercial high-purity D4 whichmainly contains only fractions of D3 and D5, but hardly any D6 and no D7and no D8.

The second start mixture is also commercial D4, but of less purity. Thisstart mixture substantially contains D3, D4, D5 and small amounts of D6and D7. This mixture constitutes the main amount in the feedstockmaterial.

The third polyalkylsiloxane mixture is also a distillate ofpolyalkylsiloxanes. The start liquid is here an intermediate fractionwhich in the multistage conventional fractionizing distillation isremoved before the last distillation step. This polyalkylsiloxanemixture consists essentially of D5: (50-85% by wt.), D6: (10-40% by wt.)and D7: (0.5-5% by wt.).

The start mixtures are mixed in the liquid phase with formation of atotal mixture in such a mixing ratio that a feedstock material evolveswith the desired fractions of additional components. The fraction of thethird start mixture in the total mixture is between 0.1% by wt. and 3%by wt.

As can clearly be seen, admixtures of D3 and D6 with a weight ratiomD3/mD6 in a range between 0.01 and 100, particularly between 0.10 and10, lead to homogeneous soot bodies on condition that the weightfraction of D7 is at least 20 ppm at the same time. If these weightratios are exceeded or not reached, the number of inhomogeneities willrise and the quality of the synthetic quartz glass will thus decline.For instance, the qualitative results of Table 1 show that when afeedstock material is used in the form of a mixture containing, apartfrom D4, a suitable quantitative proportion of thepolyalkylcycosiloxanes D3, D6, and D7, one will obtain a quartz glass inan economic way that has a uniform layer structure with a minor layerformation.

Moreover, it has been found that the sum of the weight fractions of thecomponents mD3 and mD6 is preferably in a range between 200 and 20,000wt. ppm and that the weight fraction mD7+ should be at least 20 ppm,preferably at least 50 wt. ppm, to achieve an optimal homogeneity of thequartz glass body. Deviations from the selected range and/or theselected sum of the weight fractions will each time reduce thehomogeneity of the quartz glass. Particularly distinguished were thesamples having an mD3/mD6 ratio between 0.1 and 40 and a D7 fraction of20 wt. ppm and 25 wt. ppm, respectively.

The soot bodies produced according to the prior art and/or the methodaccording to the invention can have a density that is between 25-32% ofthe density of quartz glass. The achieved density depends inter alia onthe distance of the burners from the deposition surface, the settemperature, the stoichiometry of the gases and the geometry of thedeposition burners. Different density curves within the soot body, e.g.linear, ascending or descending radial density curves in soot bodies,can be obtained by varying these factors. To analyze the densitydistributions, the local density of a soot body was determined withknown methods at about 700 measurement points. To this end about 50cross section images are made by means of CT methods over the length ofthe soot body, each of the images showing a section transverse to thelongitudinal axis of the soot body. To determine the radial densitycurve, 14 approximately equidistant measurement points are recorded ineach of the 50 CT sections. With this method the respective radialdensity curve can be determined along a sectional area through the sootbody and also a density profile along the longitudinal axis of the sootbody.

As has been explained, it seems probable that the use of the SiO₂feedstock material with additional components of different moleculesizes and reactivities leads to a broadening of the particle sizedistribution. To illustrate the different molecule sizes, FIG. 7 shows aconstitutional formula of the molecule D3, and FIG. 8 a constitutionalformula of the molecule D7.

At a molecular level SiO₂ agglomerates and aggregates of different sizesare created, the size distribution thereof covering a broader range. Thereal size distribution of the soot particles is broader than a sizedistribution of SiO₂ soot particles made from pure D4. The broaderparticle size distribution makes it possible to form a more uniform fillfor the reason that cavities are filled more uniformly. This results ina lower density variation—under macroscopic aspects—within the SiO₂ sootbody. Thus, the densities determined within the scope of the 700measurement points reflect the macroscopic result of the microscopicallybroader particle size distribution.

The mean value of the density M is obtained by forming the mean of all50 measurement points whose geometric position varies along thelongitudinal axis of the soot body, but not their geometrical distancefrom the central axis. In the case of average soot bodies, 50 crosssections are made through the soot body by way of the computedtomography method, so that the mean value of the density follows fromaveraging 50 density measurements each time. In general, the mean valuesof the density are each distributed in a normal way so that a width acan be determined. To determine the radial density profile, 14measurement points are determined in each of the 50 sections: the radialdistance of said points from the center point of the soot body is hereincreasing. The variance δ of the width a of the mean value M thuscontains a statistics of 14 points.

In soot bodies that have or should have a constant density distribution,the mean value M of the density of the soot body and the width a of themean value of the densities over 50 measurements can be determined onthe basis of the measurement data obtained. The value a thereby showshow strongly the density varies at a predetermined distance from thedeposition surface along the longitudinal axis of the soot body. Inaddition, it is also possible to calculate the variance δ of the width aon the basis of the 14 measurement points that were determined forrecording the radial profile.

To make the prior art comparable with the measurements carried outaccording to the invention, several soot bodies were measured. Theattempt was made to keep the boundary conditions, such as distance fromthe burner, burner temperature and combustion gas stoichiometry, asconstant as possible because these have a great influence on therespective density of the soot bodies. Soot bodies were each timeproduced that had a linear density profile and reached a density between25% and 32% of the density of quartz glass. When known methods ormaterials were used, one obtained the following measurement values forthe width σ and the variance δ of the density of the soot body:

σ_(StdT)=0.4% and

δ_(StdT)=0.025%.

The values are each indicated in the relative density based on thedensity of quartz glass.

The addition of additional components D3, D6 and D7 in a SiO₂ feedstockmaterial that is otherwise predominantly constituted by D4 leads to abroadening of the particle size distribution. This broadening of theparticle size distribution has the effect that the density variations inthe soot body get smaller per se. Astonishingly, it has also been foundthat the variation of the density variations was reduced. One obtained,on average, the following values for the width σ and the variance δ ofthe density of the soot body:

σ_(Inv)=0.37% and

δ_(Inv)=0.023%.

In this instance, too, the values are each indicated in the relativedensity based on the density of quartz glass. The use of the methodaccording to the invention thereby leads to a reduction of the measureddensity variations by up to 9%. Astonishingly, the variation of thewidth is also reduced by up to 8%. This reduction of the variation ofthe variation width of the mean densities results in a much morehomogeneous quartz glass than is known in the prior art.

The measurement results listed in Table 1 are also reflected in thediagram of FIG. 2. In the ordinate, the number “A” of the layers in thesoot body per length unit (in cm) is plotted, as can be determined fromthe CT measurements, and in the abscissa the ratio mD3/mD6 inlogarithmic plot in the range of 0.1 to 1000.

The sum weight (mD3+mD6) varies in these measurements, also the weightfraction mD7+, so that possibly existing relations between thecomposition and the product quality may be covered by other effects.Nevertheless, the tendency can be determined in general andqualitatively that with respect to the SiO₂ soot quality the ratiomD3/mD6 yields a relatively pronounced optimum in the range around 5 to10, which rapidly declines towards smaller or greater ratio values.

It is only possible to implement the advantages according to theinvention within a restricted composition window. The effect will fadewhen the weight ratios vary beyond the claimed range.

FIGS. 3 and 4 show layer images that were made by X-ray computedtomography methods. A cross section through a cylinder-shaped quartzglass body can each time be seen. The section is here made in adirection transverse to the longitudinal axis of the quartz glass body.As explained, the amorphous SiO₂ particles are deposited on a depositionsurface. Said deposition surface can be seen in each of said FIGS. 3 and4 as a white ring in the center of the quartz glass body that isotherwise shown in a grayish color. The SiO₂ particles are depositedlayer by layer under rotation of the rod-shaped deposition surface.

The CT image of FIG. 3 shows the SiO₂ soot tube produced by using a SiO₂feedstock material 105 according to sample no. 5 of Table 1. In thisimaging technique, regions of a relatively high density are shown asbright surface regions. It is evident that the density is uniformlydecreasing from the inside to the outside. Radial layers are hardlyvisible. By comparison, FIG. 4 shows a CT image of a soot tube producedaccording to the prior art with a commercial feedstock material.Radially extending layers can be detected by way of brightnessdifferences. The smaller inner diameter in the sample according to FIG.4 is due to the use of a carrier having a smaller outer diameter in thedeposition process for producing the SiO₂ soot tube. Density variationscan particularly be detected in an easy and clear way by using X-rayradiation for visualizing the cross section of the quartz glass body. Itis obvious that the quartz glass body that was produced according to theknown method has a plurality of concentrically structured densityvariation rings. Such types of density variations may have drawbacksparticularly when the quartz glass is used for optical fibers.

By contrast, FIG. 3 clearly shows that there are no longer anyconcentric rings that would hint at density variations. An opticalcontrol of the images made by computed tomography already reveals thatquartz glass bodies that show a very uniform homogeneity can be producedby using a polysiloxane mixture which contains the additional componentsD3, D6 and D7 in predetermined amounts and weight ratios. The uniformhomogeneity of the quartz glass directly follows from the homogeneousdistribution of the SiO₂ particles in the soot body. The transition fromthe SiO₂ soot body to the synthetic quartz glass takes place undersupply of thermal energy. Within the scope of this method step calledvitrification, defects or density variations that are present in theSiO₂ soot body manifest themselves in the quartz glass. The use ofpolyalkylsiloxanes with additional components leads, as has beenexplained, to a broadening of the particle size distribution. Thus thevariance of the density variations of the soot body can be reduced toonly δ_(Inv)=0.023%. Such homogeneous SiO₂ soot bodies can then beconverted into also extremely homogeneous quartz glass bodies byvitrification.

FIG. 6 shows the system 100 for producing quartz glass which uses themethod according to the invention. For this purpose the system 100comprises a storage tank 110 from which the liquid SiO₂ feedstockmaterial 105 is pumped by means of a pump (not shown) into a pre-heatingdevice 115. With the help of known methods the liquid SiO₂ feedstockmaterial 105 is heated up in the pre-heating device 115 to an elevatedtemperature. After having flown through the pre-heating device 115, theliquid SiO₂ feedstock material 105 is pumped into the expansion chamber125 of the vaporizer 120. As will still be explained in more detail, thetransition of the liquid SiO₂ material into the gaseous SiO₂ feedstockvapor 107 takes place in the expansion chamber 125. The SiO₂ feedstockvapor 107 flows via a line 130 to the burner 140 where a pyrolytic orhydrolytic conversion of the SiO₂ feedstock vapor into SiO₂ particlestakes place.

The pre-heating device 115 has an inlet 116 and an outlet 117. The SiO₂feedstock material 105 is fed through the inlet 116 into the pre-heatingdevice 115. The SiO₂ feedstock material 105 is heated within thepre-heating device 115. This can be done by using a hot oil system or anelectric heating element in the walls of the pre-heating device. To heatup the liquid SiO₂ feedstock material 105 in a uniform manner whileavoiding hot regions, it has turned out to be advantageous when thepre-heating device 115 comprises a flow channel which is surrounded byhot oil channels. The liquid-to-liquid heat transfer that can thereby berealized achieves a uniform heating of the liquid SiO₂ feedstockmaterial 105. This type of uniform heating ensures that there is notemperature-induced chemical conversion of the D3, D6 or D7 molecules.The heated liquid SiO₂ feedstock material 105 is discharged from thepre-heating device 115 into the expansion chamber 125 through a feedline 145.

The expansion chamber 125 defines an inner volume for the free expansionof the SiO₂ feedstock vapor. To achieve such a vaporization of theliquid SiO₂ feedstock material into the gaseous feedstock vapor, thetemperature of the liquid SiO₂ feedstock material is raised in thepre-heating device 115 above the boiling point of the SiO₂ feedstockmaterial at the operating pressure of the expansion chamber. A preferredoperating temperature for the pre-heating device 115 is about 220° C.The boiling point of D4 at atmospheric pressure is about 175° C. Toavoid a situation where the liquid SiO₂ feedstock material boils at 220°C., a back-pressure is needed in the pre-heating device 115 of about 100kPa. The liquid reactant is thereby kept as an undercooled (compressed)liquid in the pre-heating device 115.

As illustrated in FIG. 6, the liquid SiO₂ feedstock material flows fromthe pre-heating device 115 through the feed line 145 into the interiorof the expansion chamber 125. The pre-heating device 115 heats theliquid SiO₂ feedstock material 105 to an adequate degree, so that itvaporizes almost completely while its pressure drops upon entry into theinner volume of the expansion chamber 125. Such an immediatevaporization will only take place if the pre-heating device 15 hasraised the temperature of the liquid SiO₂ feedstock material above theboiling point of the SiO₂ feedstock material at the operating pressureof the expansion chamber 125. Thus the amount of the SiO₂ feedstockmaterial 105 which vaporizes immediately depends on the heating quantitysupplied to the liquid SiO₂ feedstock material in the pre-heating device115.

To achieve a particularly uniform and complete vaporization also of thehigher-boiling portions of the SiO₂ feedstock material, it isadvantageous when a heated gaseous diluent is additionally introducedinto the chamber. The SiO₂ feedstock material comprises a plurality ofpolyalkylsiloxanes, particularly D3, D4, D6 and D7. Each of the saidpolyalkylsiloxanes has a different boiling temperature and a differentvapor pressure, as can be seen in Table 2.

TABLE 2 Short Molec. Molec. T_(b) *¹⁾ P_(V)*²⁾ Chemical Designation NameFormula Weight [° C.] [Pa] Hexamethylcyclotri- D3 C6H18 222 134 1700siloxane O3Si3 Octamethylcyclotetra- D4 C8H24 297 175 140 siloxane O4Si4Decamethylcyclopenta- D5 C10H30 371 211 33 siloxane O5Si5Dodecamethylcyclohexa- D6 C12H36 444 245 5 siloxane O6Si6Tetradecamethylcyclohepta- D7 C14H42 519 270 siloxane O7Si7Hexadecamethylcycloocta- D8 C16H48 593 290 siloxane O8Si8 *¹⁾ T_(b),boiling point at 760 mmHg *²⁾P_(v;) vapor pressure at 24° C.

To maintain the weight ratios of the additional components, it isimportant that the SiO₂ feedstock material vaporizes completely andcongruently. This requires that the supplied amount of heat and thesupplied amount of diluent are so great that even D7, which has thehighest boiling temperature, can vaporize. In the pre-heating device115, heat is supplied to the SiO₂ feedstock material to heat the same.At temperatures of about 250° C., however, gels may possibly form,especially in the high-boiling polyalkylsiloxanes, D7, D8 or D9. Toprevent those high-boiling elements from forming gels, it isadvantageous when within the vaporization process a diluent 152 issupplied, preferably a gaseous diluent heated to the desiredvaporization temperature. For this purpose the gaseous diluent 152 mayflow through a corresponding media line 150 from a storage container 151into the expansion chamber 125.

Especially nitrogen has turned out to be advantageous as diluent 152.Other diluents, e.g. argon or helium, can also be used if this isdesired. These are gases that show an inert behavior with respect topolyalkylsiloxanes, so that oxidation, polymerization or decompositionreactions between the liquid and the carrier gas, especially underpressure and at elevated temperature, and thus a non-reproducible changein the composition of the feedstock material are avoided. The partialpressure of the liquid SiO₂ feedstock material in the expansion chamber125 is reduced by supplying the diluent, and the dew point thereof isthereby lowered. As a result, the SiO₂ feedstock material need not beheated at high temperatures in the pre-heating device 115. Rather,temperatures between 150° C. and 230° C. are enough to ensure a completeconversion of the SiO₂ feedstock material into the SiO₂ feedstock vapor.It is here the aim that the vaporization of the SiO₂ feedstock materialcomprises an injection phase in which the feedstock material is atomizedin liquid form into fine droplets, and a vaporization phase in which thefine droplets are rapidly and efficiently vaporized completely bycontact with a hot carrier gas, but without contact with walls.

FIG. 6 illustrates the vaporization according to the invention. Theheated-up SiO₂ feedstock material 105 is supplied through the feed line145 to the expansion chamber 125. At the end of the feed line 145 in theinterior of the expansion chamber 125, the feed line 145 comprises anozzle-shaped atomizing nozzle 128. With the help of the atomizingnozzle 128 the liquid SiO₂ feedstock material 105 is atomized into finedroplets having a mean diameter of less than one μm, preferably between0.5 μm and 20 nm (d₅₀ value). Due to the pressure drop occurring uponexit out of the atomizing nozzle 128, a substantial part of the liquidfeedstock material is transferred into the gas phase. In addition, anitrogen stream preheated to about 200° C. to about 230° C. is passedthrough the media line 150 into the expansion chamber 125.Advantageously, the nitrogen stream has a temperature that correspondssubstantially, i.e. +/−10′C, to the temperature of the liquid SiO₂feedstock material 105. According to the invention the nitrogen streamflows opposite to the spray direction of the liquid SiO₂ feedstockmaterial 105 so as to ensure strong intermixing and adequate heattransfer. Due to the combination of the two vaporization principles,pressure drop and vaporization by means of the gaseous nitrogen, theliquid SiO₂ feedstock material 105 is fully converted into the gaseousSiO₂ feedstock vapor 107. It is not intended that parts of the liquidSiO₂ feedstock material 105 deposit on the walls of the expansionchamber 125 and/or are vaporized thermally at said place. The gaseousSiO₂ feedstock vapor 107 flows off through the line 130 to the burner140. In the burner 140, the SiO₂ feedstock vapor 107 is converted bypyrolysis, oxidation or hydrolysis into SiO₂ particles 148, also calledSiO₂ or soot or SiO₂ soot.

The size range of the droplets depends on many parameters. Apart fromthe rheological properties of the liquid and the geometry of theatomizing nozzle 128, these are particularly the exit velocity of theliquid out of the atomizing nozzle, which velocity is substantiallydetermined by the pressure difference. Within the said pressuredifference range the exiting liquid jet disintegrates into fine dropletswith a narrow droplet size distribution due to the turbulent flow.

In another method variant, it is intended that forming the stream of theSiO₂ feedstock material comprises generating a first gas stream byvaporizing a mixture of polyalkylsiloxanes, which contains D3, D4, D5and D6, and generating a second gas stream by vaporizing a secondmixture of polyalkylsiloxanes, which substantially contains D3, D4, D5,D6 and D7, and combining the gas streams before or during method step(C). The vaporization parameters, such as vaporization temperature andvaporization rate, can be adapted individually to the components to bevaporized and can be optimized by using separate vaporizers fordifferent components of the feedstock material.

LIST OF REFERENCE NUMERALS

-   100 system-   105 SiO₂ feedstock material-   107 SiO₂ feedstock vapor-   110 storage tank/storage container-   115 pre-heating device-   116 inlet-   117 outlet-   120 vaporizer/vaporizer system-   122 liquid pump-   123 flow meter-   124 MFC (mass flow controller)-   125 expansion chamber/vaporization chamber-   126 line-   127 flow distributor-   128 atomizing nozzle-   130 line-   140 burner/flame hydrolysis burner-   141 burner block-   142 movement of 140-   143 burner flame-   145 feed line-   148 SiO₂ soot-   150 media line-   151 storage container-   152 diluent-   160 deposition surface/carrier tube-   161 longitudinal axis of 160-   200 soot body

1. A method for producing synthetic quartz glass, said methodcomprising: (a) providing a liquid SiO₂ feedstock material, whichcontains octamethylcyclotetrasiloxane D4 as the main component, (b)vaporizing the SiO₂ feedstock material into a feedstock material vapor,(c) converting the feedstock material vapor into SiO₂ particles, (d)depositing the SiO₂ particles (148) on a deposition surface so as toform a porous SiO₂ soot body, (e) vitrifying the SiO₂ soot body so as toform the synthetic quartz glass, wherein the liquid feedstock materialcontains additional components comprising a D3 component consisting ofhexamethylcyclotrisiloxane D3 and/or a linear homolog thereof with aweight fraction mD3, a D6 component consisting ofdecamethylcyclohexasiloxane D6 and/or a linear homolog thereof with aweight fraction mD6, and a D7+ component consisting oftetradecamethylcycloheptasiloxane D7 and/orhexadecamethylcyclooctasiloxane D8 and/or linear homologs thereof with aweight fraction mD7+, wherein a weight ratio mD3/mD6 is in a rangebetween 0.05 and 90 and the weight fraction mD7+ is at least 20 wt. ppm.2. The method according to claim 1, wherein the ratio mD3/mD6 is in arange between 0.1 and
 40. 3. The method according to claim 1, whereinthe weight fraction mD7+ is in a range between 30 and 100 wt. ppm. 4.The method according to claim 1, wherein the sum of the weight fractionsmD3+mD6 is in a range between 200 and 20,000 wt. ppm.
 5. The methodaccording to claim 1, wherein mD3 is in a range between 200 wt. ppm and15,000 wt. ppm, and mD6 is in a range between 50 wt. ppm and 2,000 wt.ppm.
 6. The method according to claim 1, wherein said vaporizingcomprises: heating the liquid SiO₂ feedstock material, introducing theheated SiO₂ feedstock material into an expansion chamber, so that atleast a first part of the SiO₂ feedstock material vaporizes due to apressure drop, mixing the SiO₂ feedstock material with a heated diluent,so that at least a second part of the SiO₂ feedstock material vaporizesdue to a decrease in the dew point.
 7. The method according to claim 6,wherein said introducing of the heated SiO₂ feedstock material into theexpansion chamber comprises an injection phase in which the SiO₂feedstock material is atomized in liquid form into fine droplets, thedroplets having a mean diameter of less than 5 μm.
 8. The methodaccording to claim 6, wherein the fine droplets contact the heateddiluent in the form of a hot carrier gas in the chamber, which is keptat a temperature in a range of 150° C. to 230° C.
 9. The methodaccording to claim 8, wherein during introduction of the SiO₂ feedstockmaterial into the expansion chamber a concentration detector measures acomposition of the SiO₂ feedstock material.
 10. The method according toclaim 1, wherein said providing a liquid SiO₂ feedstock materialaccording to method step (a) comprises: providing a first mixture ofpolyalkylsiloxanes in which D3, D4, D5, D6 and linear homologs thereofaccount for at least 99 wt. %, providing a second mixture ofpolyalkylsiloxanes in which D7 accounts for an amount of at least 5,000wt. ppm, producing a blend from the first and second mixtures ofpolyalkylsiloxanes prior to or during vaporization according to methodstep (b) in a mixing ratio such that the blend has contains D7 and/or D8and/or linear homologs of D7 or D8 such that the blend has a secondweight fraction of D7 and/or D8 and/or linear homologs of D7 or D8 of atleast 20 wt. ppm in the total blend.
 11. The method according to claim1, wherein the SiO₂ particles are deposited on the deposition surfaceaccording to method step (b) and the feedstock material vapor issupplied to a plurality of deposition burners arranged in a burner row,which are moved jointly and reversingly along the deposition surface.12. A method comprising: producing a synthetic quartz glass according toclaim 1; and producing an optical fiber from said synthetic quartzglass.
 13. The method according to claim 1 wherein a sum of the weightfractions mD3+mD6 is in a range between 500 and 15,000 wt. ppm.
 14. Themethod according to claim 7, wherein the mean diameter of the dropletsis less than 2 μm.
 15. The method according to claim 10, wherein in thesecond mixture of polyalkylsiloxanes D7 accounts for an amount of atleast 10,000 wt. ppm.
 16. The method according to claim 10, wherein inthe second mixture of polyalkylsiloxanes D7 accounts for an amount of atleast 20,000 wt. ppm.